This invention relates generally to vehicle transfer cases having a plurality of outputs and more particularly to a transfer case providing full-time four-wheel drive with a mode shift selection arrangement wherein viscous clutch means, operable to modify the torque division between the front and rear axles in the four-wheel drive mode, is disconnected in the two-wheel drive mode.
A number of prior vehicle drive train transfer cases have used planetary gear sets to selectively apply drive torque to one or two output shafts providing two-wheel or four-wheel drive. Reference may be made, for example, to U.S. Pat. No. 4,074,591 issued Feb. 21, 1978 to W. M. Dick and U.S. Pat. No. 4,677,873 issued July 7, 1987 to R. E. Eastman et al. which discloses a transfer case of this type.
It is common for such transfer cases, when operating in four-wheel drive mode during constant-speed driving, to provide means for selectively interlocking axles driven thereby preventing any slippage with respect to each other. It is also common for such transfer cases, when operating in four-wheel drive mode under certain other conditions, to provide means for differential action between respective axles permitting torque division and speed variations between the axles and thereby prevent potentially damaging torque buildup in the drive train.
Differential action is typically provided by a differential mechanism interconnecting one axle with the other. The above mentioned Eastman et al., patent discloses a compact dual planetary inter-axle differential without an annulus gear uniquely adapted for use with the present invention.
The U.S. Pat. No. 4,373,604 issued Feb. 15, 1983 to Lunn et al. and U.S. Pat. No. 4,381,828 issued May 3, 1983 to Lunn et al. disclose transfer cases adapted for use in four wheel drive vehicles having two-wheel drive and four-wheel drive operating modes. The Lunn et al., patents have an inter-axle differential incorporating a viscous fluid clutch which allows the front and rear sets of wheels to rotate independently under normal driving conditions but which transfers torque to the set of wheels which has higher traction when the other set of wheels slips. In full-time four-wheel drive transfer cases it is imperative that the viscous fluid clutch be disconnected upon shifting to the two-wheel drive mode to obviate a driving connection through the viscous clutch between the front and rear axles.